In the field of telecommunications it is adequately known that a caller can make use of the “callback” service feature if the called subscriber is not present.
Various implementations are known for this, such as for example “Completion of Calls to Busy Subscribers”/“Completion Call on No Reply” (CCBS/CCNR), which are used with (ISDN) and the ISDN-based QSIG (“Signaling at the Q reference point”) protocol.
A callback option is also known from mobile communication, with which missed calls are offered as a menu for a call(back). (High) charges are incurred for the party calling back in mobile communication in particular.